1. Field of the Invention
The invention relates generally to the field of polarized light sources, such as polarization conversion systems (PCS) for viewing displays directly or by projection. More particularly, the invention relates to a polarized light source with a reverse optical path to enhance brightness.
2. Description of the Related Art
Many displays used in projection and direct viewing systems operate on the basis of polarization. Such displays include reflective displays such as LCoS (Liquid Crystal on Silicon), super twisted nematic (STN), and ferroelectric (FLC) as well as transmissive displays, such as thin film transistor (TFT), poly-silicon (p-si), and Silicon-on-Insulator (SOI). These displays can produce a high resolution image by changing the polarization state upon reflection or transmission of incident light. In an LCoS display, for example, in the dark state, a pixel reflects all light with substantially no change in polarization. In the bright state, the pixel rotates the polarization state of reflected incident light to the corresponding orthogonal state. By illuminating the display with polarized light and then filtering out nearly all reflected light of that polarization, the display image can be viewed by the human eye or projected onto a viewing screen.
In a single panel projection system, the display is illuminated with short bursts of red, green, and blue light while the display is synchronized to the pulsed light source to reflect the appropriate color component of the image. A white light or other color light burst can also be used alone or in combination with the red, green, and blue light. The short bursts can come from a color wheel or from pulsed LEDs (Light Emitting Diodes). The rapidly alternating red, green, and blue images are blended in human perception to form the full-color image of the display. However, the display can also be illuminated with monochromatic light for data or targeting displays. Such displays are used, for example in helmet, windshield, and visor projection systems as well as in small portable projectors and handsets for private display viewing and for virtual reality systems.
Because most conventional low cost light sources produce light with mixed polarization states, the light is typically analyzed by a PBS (Polarizing Beam Splitter). Light of one polarization direction (usually S-polarization) propagates through the PBS, while light of the orthogonal polarization direction (usually P-polarization) is reflected by the PBS. Another common approach is to use a polarizing filter that absorbs light of one polarization direction. Normally in such systems without a polarization conversion system, half of the light is lost either from reflection or absorption. This results in a dimmer display or requires a brighter light source. In a projector, a dimmer display is more difficult to view, while a brighter light source increases the power consumption and the cost of the projector system. Due to the additional heat typically generated, the brighter light source may require a larger housing to provide enough room for cooling or to accommodate a fan to cool the light source. The fan adds additional cost, power consumption and noise.
In order to increase efficiency, a multi-PBS can be used instead of the simple single PBS. The multi-PBS has a two dimensional array of small polarizing beam splitters and associated lenses. The beam splitters and lenses are precisely aligned so that the output of the multi-PBS is substantially collimated and has a single polarization state. The multi-PBS converts almost all of the input light to the same polarization state. However, it is expensive to produce due to the complex structures required and the precision required to align each of the PBS structures with each lens. The multi-PBS therefore increases the cost of the projection system.
Another disadvantage of the multiple and single PBS systems comes from a difference in the transmissivity over different incident angles in a typical PBS between horizontal and vertical axes. In most cases, the PBS will transmit light received at a greater range of incident angles in one axis than in the other. As a result, the PBS may be more efficient in the vertical direction than in the horizontal direction. The efficiency of the PBS can be increased by spreading the light in that direction, however, the angular intensity distribution for a conventional PCS (polarization conversion system) is point symmetrical around the center.